The discovery of superconducting ceramic oxides has fueled a tremendous effort to fabricate these oxides into high performance films and coatings. High temperature superconducting (HTSC) film fabrication methods can be largely divided into two areas: physical and chemical methods.
Physical methods include reactive evaporation, magnetron sputtering, e-beam deposition and laser ablation. While physical deposition methods form high quality films, these deposition techniques typically have very slow formation rates, and require high vacuum environments so that they require expensive equipment. In addition, the techniques are best suited for thin-film fabrication. For these reasons, physical deposition methods are extremely difficult to scale up to multi-meter lengths required for electrical or magnetic applications.
Chemical methods are largely based upon thermally activated chemical reactions of precursor compounds during film formation. Chemical film fabrication methods involve a precursor which is deposited onto a substrate and later transformed through thermal and chemical means to a film having the desired composition and phase.
Films may be prepared using metalorganic chemical vapor deposition (MOCVD), in which precursor films are deposited from metalorganic precursors having a high vapor pressure. Metal-organic solution deposition (MOD) processes involve the deposition of a precursor film from a condensed phase precursor. The precursor film is then heated and converted into the final ceramic in a separate heat treatment.
MOD processes are widely used in industry for the deposition of ceramic films. The process is ideally suited for the rapid, inexpensive deposition of films on large or continuous substrates. Other advantages of the MOD process include easy control of metal composition and homogeneity, short processing time, low capital equipment cost and low precursor cost.
Typically in MOD processes, metal carboxylates of carboxylic acids, alkoxides, or partially hydrolyzed alkoxides are dissolved in organic solvents and the resultant solution is deposited onto a substrate by dipping or spin coating. The precursor films produced by these coating processes are transformed into metal compound-containing coatings by heat treatment, which most commonly includes a series of distinct heating steps. While chemical methods represent versatile and inexpensive methods of film fabrication with potential for high speed production, they are very sensitive to secondary reactions which may be deleterious to final superconducting properties. For example, in the deposition of materials such as YBa.sub.2 Cu.sub.3 Oy, such processes are highly susceptible to the intermediate formation of barium carbonate (BaCO.sub.3). The stability of BaCO.sub.3 requires high processing temperatures (&gt;900.degree. C.) and extended processing times in order to decompose the barium carbonate and obtain the oxide superconductor. The extreme reaction conditions result in film reaction with the substrate, poor texture of the oxide superconductor and incomplete formation of the oxide superconductor phase.
Chan et al. in Appl. Phys. Lent. 53(15):1443 (October 1988) discloses a hybrid process, known as an ex situ process, which includes the physical deposition of a precursor film which is then processed outside of the physical deposition chamber by conventional chemicothermal processes. This PVD process (BaF.sub.2 ex situ process) separates the deposition and conversion steps. This process involves codeposition of CuO, Y.sub.2 O.sub.3, and BaF.sub.2 in the correct stoichiometric uniformly on the substrate. The film is then converted under conventional heating conditions into the oxide superconductor by annealing in the presence of water vapor. The limitations of physical deposition methods described above remain, however. Chan et al observed that improved electrical performance was obtained by increasing the P.sub.O2 and decreasing the P.sub.HF during the anneal step.
Cima et al. in U.S. Pat. No. 5,231,074, report the MOD preparation of Ba.sub.2 YCu.sub.3 O.sub.7-x, (YBCO) oxide superconductor films having improved electrical transport properties by MOD using metal trifluoroacetates on single crystal SrTiO.sub.3 and LaAlO.sub.3. The films of a thickness of about 0.1 .mu.m possessed critical transition temperatures of about 90 K and zero field critical current densities of greater than 10.sup.6 A/cm.sup.2 at 77 K.
In addition, the superconducting performance of epitaxial Ba.sub.2 YCu.sub.3 O.sub.7-x films prepared using the process described in U.S. Pat. No. 5,231,074 has been found to depend on film thickness. Electrical performance drops off dramatically as film thickness increases from 0.1 .mu.m to 1.0 .mu.m. Although thinner films have routinely been prepared with critical current densities greater than 10.sup.6 A/cm.sup.2, application of conventional chemical processing techniques in the preparation of films with a thickness near 1.0 .mu.m never yielded results close to this level of performance. For example, a MOD process using metal trifluoroacetates has been used to prepare thin (70-80 nm) YBa.sub.2 Cu.sub.3 O.sub.y (YBCO) films (where y is a value sufficient to impart superconductivity at temperatures of at least 77 K) with T.sub.c &gt;92 K and J.sub.c &gt;5.times.10.sup.6 A/cm.sup.2 (77 K, self field); however, it has not been possible to prepare much thicker films possessing similar properties. Indeed, prior to the development of the processing techniques described in this patent application no solution-based deposition process had been demonstrated that produced high J.sub.c films with thicknesses of over 0.5 .mu.m.
Thicker oxide superconductor coatings are needed in any application requiring high current carrying capability such as power transmission and distribution lines, transformers, fault current limiters, magnets, motors and generators. Thicker oxide superconducting films are desired to achieve a high engineering (or effective) critical current (J.sub.c), that is, the total current carrying capability divided by the total cross sectional area of the conductor including the substrate.
It is desirable that oxide superconducting coatings greater than 0.5 .mu.m in thickness have high critical current densities. There is a need for fabrication techniques which may be used to prepared these thick oxide superconductor films and coatings with superior electrical performances.